[Ironhawk]

1. Libraries are a level-0 magic site, which means as soon as you capture the province it will be visible to you. Right clicking on it will say "Allows you to recruit sages". And when you check the recruitment screen for that province you will in fact find the Sage commander. Build a Laboratory there to enable recruitment of the Sage. Then, to protect your investment it is usually wise to build a Fort as well. As you play more you will encounter many magic sites of this kind which allow you to recruit different kinds of mages (tho Sages are the best researchers).

2. The term "Fortress" is the general term in dom2 to describe any fortification. Some people use castle and fort terms interchangably. The kind of fort that your nation will build is the one that you select during the Pretender Creation process ie (watchtower, mausoleum, wiz tower, hill fort, etc).